Implement clock controller

[moojek]

Initial version - since I was lazy and it took very long to actually write this, I forgot exact requirements we made for this.
I'd like some input particularly on:

• in what units we want to operate the clock;
• do we want to support some kind of queuing or only one timer at a time;
• should it support custom callbacks or only one predefined;
• when requested should it run once or periodically.